Implementing typescript support

[sanathusk]

The PR includes that allow typescript support in svelte components.

[codecov]

Codecov Report

Base: 83.61% // Head: 83.84% // Increases project coverage by +0.23% 🎉

Coverage data is based on head (09be8e5) compared to base (919a81e).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2919      +/-   ##
==========================================
+ Coverage   83.61%   83.84%   +0.23%     
==========================================
  Files          31       32       +1     
  Lines        1050     1065      +15     
==========================================
+ Hits          878      893      +15     
  Misses        172      172              

Impacted Files	Coverage Δ
src/lib/window.js	100.00% <0.00%> (ø)
src/lib/reporter.js	100.00% <0.00%> (ø)
src/core/plugins/index.js	100.00% <0.00%> (ø)
src/core/helpers/session.js	100.00% <0.00%> (ø)
src/core/core.js	72.14% <0.00%> (+0.61%)	⬆️

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[NeoTheThird]

Take a look at @maciek134's requests, then it looks good!